The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Oxides of nitrogen, NOx, are known by-products of combustion. NOx is created by nitrogen and oxygen molecules present in engine intake air disassociating in the high temperatures of combustion, and rates of NOx creation include known relationships to the combustion process, for example, with higher rates of NOx creation being associated with higher combustion temperatures and longer exposure of air molecules to the higher temperatures. Reduction of NOx created during the combustion process are priorities in engine design and engine and exhaust aftertreatment control systems.
NOx molecules, once created in the combustion chamber, can be converted back into nitrogen and oxygen molecules in devices known in the art within the broader category of aftertreatment devices. However, one having ordinary skill in the art will appreciate that aftertreatment devices are largely dependent upon operating conditions, such as device operating temperature driven by exhaust gas flow temperatures.
Modern engine control methods utilize diverse operating strategies to optimize combustion. Some operating strategies, optimizing combustion in terms of fuel efficiency, include lean, localized, or stratified combustion within the combustion chamber in order to reduce the fuel charge necessary to achieve the work output required of the cylinder. While temperatures in the combustion chamber can get high enough in pockets of combustion to create significant quantities of NOx, the overall energy output of the combustion chamber, in particular, the heat energy expelled from the engine through the exhaust gas flow, can be greatly reduced from normal values. Such conditions can be challenging to exhaust aftertreatment strategies, since aftertreatment devices frequently require an elevated operating temperature, driven by the exhaust gas flow temperature, to operate adequately to treat NOx emissions.
Aftertreatment devices are known, for instance, utilizing catalysts capable of storing some amount of NOx, and engine control technologies have been developed to combine these NOx traps or NOx adsorbers with fuel efficient engine control strategies to improve fuel efficiency and still achieve acceptable levels of NOx emissions. One exemplary strategy includes using a NOx trap to store NOx emissions during fuel lean operations and then purging the stored NOx during fuel rich, higher temperature engine operating conditions. Such purging events or regeneration events can be the result of changing vehicle operation or forced purging events. A forced purging event requires monitoring the amount of NOx stored and some mechanism or criteria to initiate the purge. For example, a NOx trap has a limited storage capacity, and sensors can be used in the exhaust gas flow to estimate NOx generation in order to estimate the NOx trap state. Once the NOx trap gets close to its full capacity, it must be regenerated with a fuel rich reducing “pulse.” It is desirable to control the efficiency of the regeneration event of the NOx trap to provide optimum emission control and minimum fuel consumption. Various strategies have been proposed.
Techniques are known for adsorbing NOx (trapping) when the air-fuel ratio of the exhaust gas flowing into the NOx adsorbent is lean and releasing the adsorbed NOx (regenerating) when the air-fuel ratio of the exhaust gas flowing into the NOx adsorbent becomes rich wherein the amount of NOx adsorbed in the NOx adsorbent may be estimated from the engine load and the engine rotational speed. When the amount of the estimated NOx becomes the maximum NOx adsorption capacity of the NOx adsorbent, the air-fuel ratio of the exhaust gas flowing into the NOx adsorbent is made rich. Determination of a regeneration phase may also be on the basis of individual operating cycles of the internal combustion engine.
It is also known to estimate how full the NOx trap is by estimating the amount of NOx flowing into the NOx trap using a NOx sensor or a pre-NOx trap oxygen sensor. It is also known to schedule regeneration based on estimations of accumulated NOx mass and engine load and speed operating condition probabilities. Increasingly stringent emission standards require NOx aftertreatment methods, utilizing, for example, a selective catalytic reduction device (SCR). An SCR utilizes ammonia derived from urea injection or recovered from normal operation of a three-way catalyst device to treat NOx. Continued improvement in exhaust aftertreatment requires accurate information regarding NOx emissions in the exhaust gas flow in order to achieve effective NOx reduction, such as dosing proper amount of urea based on monitored NOx emissions.
A NOx sensor or an oxygen sensor add cost and weight to a vehicle, and such sensors frequently require a particular operating temperature range, achieved after some warm-up time, to be functional. There exist methods to estimate engine-out NOx via detailed combustion modeling using heat release model, multi-zone combustion model and Zodovich chemical kinetic equations. This detailed modeling, although good for analysis, may not be appropriate for in-vehicle engine control module (ECM) applications because of complicated programming and calibration requirements. Additionally, such models are sensitive to sensor tolerance and aging, pose a large computational burden upon the ECM, require processing time not providing results in real-time, and do not compensate for dynamic engine conditions and transient system events affecting NOx generation.